1. Field of the Invention
This invention relates to an optical disc device to perform recording onto and reproduction from an optical disc, and in particular relates to an optical pickup in an optical disc device. More specifically, this invention relates to an optical pickup equipped with a plurality of objective lenses, so as to enable recording onto and reproduction from a plurality of types of optical discs having different optical characteristics.
2. Description of the Related Art
Various configurations have been devised for optical pickups for optical disc devices to enable recording and reproduction using a plurality of types of optical discs having different optical characteristics. In particular, due to the appearance of Blu-ray discs, which are high-density recording media, hereafter optical disc devices will be sought which are capable of recording onto and reproduction from blue-light optical discs (HD-DVD and Blue-ray discs) in addition to conventional red-light optical discs (CDs and DVDs).
In order to enable recording onto and reproduction from a plurality of types of optical discs having different optical characteristics using a single optical pickup, a major issue is how to equip the optical pickup with objective lenses. Two methods of resolving this problem are being considered. One method involves equipment with a single objective lens which is compatible with the plurality of types of optical discs having different optical characteristics. The second method involves equipment of a plurality of objective lenses, corresponding to the plurality of types of optical disc.
Next, the thickness of objective lenses corresponding to principal optical discs is explained, referring to FIG. 14. In (A) of FIG. 14, the symbol 101 denotes an objective lens compatible with DVDs and CDs. The thickness Ta of this compatible objective lens 101 is approximately 1.3 mm. The symbol 102 denotes a Blu-ray objective lens compatible with Blu-ray discs. The thickness Tb of this Blu-ray objective lens 102 is approximately 2.5 mm. The symbol 103 denotes a BD/DCD/CD compatible objective lens, which can be used with DVDs, CDs, and Blu-ray discs. The thickness Tc of this BD/DVD/CD compatible objective lens 103 is approximately 4.5 mm. When the objective lens along accounts for this 4.5 mm thickness, considering that the standing mirror is placed directly below the objective lens, there has been the problem that the thickness of the optical pickup becomes larger than the thickness of slim drives (7.3 mm) used in notebook computers, which serve as one yardstick of thinness.
Consequently, from the standpoint of reducing thickness, the second of the above methods (the method of equipping the pickup with a plurality of objective lenses) is regarded as more effective for an optical pickup configuration enabling recording/reproduction of Blu-ray discs, DVDs and CDs.
FIG. 15 is a perspective view showing an actuator for an optical pickup disclosed in Japanese Patent Laid-open No. 2003-281758. The actuator 100 is a two-axis driving device in which objective lenses 112 and 113, corresponding to optical discs with different optical characteristics, are mounted on a single movable member 111, and the movable member 111 can be driven independently in the focusing direction F and in the radial direction R. Such an actuator 100 can be applied to an optical pickup for recording onto/reproduction from a plurality of optical discs with different optical characteristics.
The actuator 100 of Japanese Patent Laid-open No. 2003-281758 is structured with the movable member 111 supported by a base 130 via a holder 114. Two objective lenses, 112 and 113, are mounted on the movable member 111, arranged in the radial direction R. The actuator 100 comprises a support member 115, connecting the movable member 111 to the holder 114, and which supports the movable member 111 moveably in the focusing direction F and in the radial direction R; first magnetic circuits 150, to move the movable member 111 in the focusing direction F; and second magnetic circuits 160, to move the movable member 111 in the radial direction R.
As the manner of arrangement of the objective lenses 112 and 113, in consideration of the fact that the data recording/reproduction region of a high-density optical disc is positioned on the inner side compared with the data recording/reproduction region of a low-density optical disc, the objective lens denoted by the symbol 112 is placed so as to be positioned on the inner side of the optical disc, as the lens for use with high-density optical discs.
Two sets of each of the first and second magnetic circuits 150 and 160 are positioned so as to surround on two sides the objective lenses 112, 113 along the tangential direction (in the direction along the circumference) T of the optical disc. Each set of the first magnetic circuits 150 comprises a focusing magnet 151 and a focusing coil 152; each set of the second magnetic circuits 160 comprises a tracking magnet 161 and a tracking coil 162. The focusing magnets 151 and tracking magnets 161 are permanent magnets; the focusing magnets 151 are provided further to the outside of the movable member 111 than the tracking magnets 161. The focusing coils 152 are provided between the movable member 111 and the focusing magnets 151; the tracking coils 162 are provided between the movable member 111 and the tracking magnets 161.
An optical pickup which uses such an actuator 100 must necessarily be configured as shown in FIG. 16. That is, below the actuator 100 are positioned a light source 140, which emits laser light L; a collimator lens 141; a standing mirror 135 to guide the laser light L to the objective lenses 112, 113; and a photodetector 143 to detect, for the laser light L reflected by and returning from the optical disc outside the drawing, the quantity of light and similar. Between the light source 140 and the standing mirror 135 is provided a half-mirror 142 to transmit laser light L from the light source 140 to the standing mirror 135 and to guide laser light L which has been reflected by and returns from the optical disc to the photodetector 143.
Laser light L emitted from the laser light source 140 passes through the half-mirror 142, is guided by the standing mirror 135 to one of the objective lenses 112 or 113, and irradiates the recording surface of the optical disc, held by the disc rotation mechanism. Laser light L reflected by the recording surface of the optical disc passes through an objective lens 112 or 113 and is reflected by the standing mirror 135, and is guided via the half-mirror 142 to the photodetector 143.
In an optical pickup using the above-described actuator of Japanese Patent Laid-open No. 2003-281758, as shown in FIG. 16, a standing mirror must necessarily be positioned below the actuator, so that in order to achieve a thin shape, the magnetic circuits, standing mirror, and other components must be made compact.
However, there are the problems that, when magnetic circuits are made compact the driving performance in the focusing direction or the radial direction declines, power consumption increases, and the tracking precision declines, so that there are difficulties attending a compact design for magnetic circuits.
Further, the size of the standing mirror depends on the effective diameter of the objective lenses; as optical design conditions, the width of the laser light beam reflected by the standing mirror must be larger than the effective diameter of the objective lens, so that reduction of the size of the standing mirror and a thin design are extremely difficult to achieve.